The calendar instituted by Julius Caesar in 45 BC was a significant improvement over the irregular Roman system used before him. However, it was still inaccurate. Its regular leap year every fourth year overestimated the length of the solar year by 11 minutes and 14 seconds, causing the vernal equinox to drift backward a full day every 128 years. By the Middle Ages, this discrepancy was remarked upon by many writers and scientists, but the science of astronomy hadn’t yet developed enough to correct it.
The Need for Calendar Reform
As astronomy advanced in the 16th century, the Council of Trent called on the Holy See to reform the calendar and return the equinox to March 21st. Pope Gregory XIII appointed a commission in 1577 to study the issue. Among its members were the Calabrian Antonio Lilio and the Bavarian Jesuit Christopher Clavius.
(Gregory XIII's Cenotaph, with Astronomers Presenting Findings)
Lilio’s Plan and Clavius' Defense
Lilio proposed removing 3 leap years every 400 years, the system that would become known as the Gregorian calendar. This adjustment prevented further drift of the equinox. His system also proposed that centenary years would only be leap years if they are evenly divisible by 400. Hence, 1600 was a leap year, but 1700, 1800, and 1900 were not; similarly, 2000 was a leap year, but 2100 will not be. Clavius defended this plan in an 800-page treatise. (Both he and Lilio have notable craters on the moon named for them; Clavius’ is the second largest on the bright side.) His arguments prevailed with the Pope, and today, this system is so accurate that the equinox will not shift by a full day again until the 47th century.
Pope Gregory XIII’s Decree: Inter Gravissimas
On February 24, 1582, Pope Gregory issued the bull Inter gravissimas, which formally established the new leap year system. However, the new calendar also had to restore the equinox to its proper place. He therefore commanded:
“Quo igitur vernum æquinoctium, quod a patribus concilii Nicæni ad XII Kalendas Aprilis fuit constitutum, ad eamdem sedem restituatur, præcipimus et mandamus ut de mense Octobri anni MDLXXXII decem dies inclusive a tertia Nonarum usque ad pridie Idus eximantur, et dies, qui festum S. Francisci IV Nonas celebrari solitum sequitur, dicatur Idus Octobris…”
“In order that the spring equinox, which was fixed by the fathers of the Council of Nicea on March 21, may be restored to this position, we command and order that ten days be removed from October of the year 1582, from the 5th through the 14th inclusively, and the day which usually follows the wonted feast of Saint Francis on the 4th shall be called the 15th…”
This effectively eliminated ten days from the calendar in October 1582.
The Impact of Calendar Reform
The Gregorian calendar was adopted almost immediately by Catholic countries, including the Spanish New World. However, Protestant nations resisted the change, with Voltaire famously quipping that the English preferred that their calendar disagree with the sun rather than agree with the Pope. The United Kingdom and its colonies, including the future United States, didn’t adopt the new system until 1752.
Orthodox nations, predominantly in Eastern Europe, held on to the Julian calendar until the early 20th century, and many Eastern Christians still use it for religious observances. Today, the Julian calendar is 13 days behind the Gregorian calendar. In some Eastern European countries, where the religious calendar is Julian, but the civil Gregorian, Christmas is celebrated six days after the civil New Year.
The Global Spread of the Gregorian Calendar
While the Gregorian calendar is now the civil calendar used by most of the world, its adoption was gradual, especially in non-Catholic regions. The reform was so successful that only minor corrections will be needed centuries from now.
